Field of the Invention
The present invention relates to a mechanism for changing an access or communication connection domain for re-routing a connection. In particular, the present invention relates to a mechanism for changing from a specific first communication domain, such as a Packet Switched (PS) domain, to a second communication domain, such as a Circuit Switched (CS) domain, in a terminating session setup procedure.
Related Prior Art
In the last years, an increasing extension of communication networks, e.g. of wire based communication networks, such as the Integrated Services Digital Network (ISDN), or wireless communication networks, such as the cdma2000 (code division multiple access) system, cellular 3rd generation (3G) communication networks like the Universal Mobile Telecommunications System (UMTS), cellular 2nd generation (2G) communication networks like the Global System for Mobile communications (GSM), the General Packet Radio System (GPRS), the Enhanced Data Rates for Global Evolutions (EDGE), or other wireless communication system, such as the Wireless Local Area Network (WLAN) or Worldwide Interoperability for Microwave Access (WiMax), took place all over the world. Various organizations, such as the 3rd Generation Partnership Project (3GPP), Telecoms & Internet converged Services & Protocols for Advanced Networks (TISPAN), the International Telecommunication Union (ITU), 3rd Generation Partnership Project 2 (3GPP2), Internet Engineering Task Force (IETF), the IEEE (Institute of Electrical and Electronics Engineers), the WiMax Forum and the like are working on standards for telecommunication network and access environments.
Generally, for properly establishing and handling a communication connection between network elements such as a user equipment (UE) and another communication equipment or user equipment, a database, a server, etc., one or more intermediate network elements such as control network elements, support nodes or service nodes are involved.
One important application is multimedia communication services. A multimedia call is a communication where, for example, audio (voice), text, video and picture are used simultaneously. Multimedia calls generally require the transmission of several different types of data (video, audio, and the like) in parallel, and these data are to be transmitted and received by various different types of communication equipments or network elements, so that it is required that plural communication protocols are negotiated and appropriate parameters for the communication are adjusted.
A current technology to merge the Internet with the cellular telecommunication world is the Internet Protocol (IP) Multimedia Subsystem IMS. IMS is a standardized architecture for operators intending to provide mobile and fixed multimedia services. IMS uses a Voice over IP (VoIP) implementation based on a 3GPP standardized implementation of Session Initiation Protocol (SIP) and runs over the standard Internet Protocol (IP). Both PS and CS communication systems are supported.
As a part of the signaling mechanisms used between the IMS and an user equipment the Session Initiation Protocol (SIP) is used. Details of the structure and procedures executed in IMS are described in the related standards and are commonly known to a person skilled in the art so that a description thereof is omitted herein for the sake of simplicity.
However, it is necessary to ensure interworking between the both CS and PS systems so that an end user experience is not jeopardized.
The IMS provides several services, such as IMS Centralized Services (ICS). ICS provides communication services such that all services, and service control, are based on IMS mechanisms and enablers. It enables IMS services when using CS access for the media bearer.
With ICS, the user services are provided by IMS. User sessions are controlled in IMS via PS or CS access. When using a CS access network, or when using a PS access network that does not support the full duplex speech component of an IMS service, the CS core network is utilized to establish a circuit bearer for use as media for IMS sessions. On the other hand, if the PS access network does support, for example, the full duplex speech component of an IMS service then standard IMS session procedures may be used.
As mentioned above, the ICS provides mechanisms to support the use of CS media bearer for IMS sessions. With ICS, IMS sessions using CS media are treated as standard IMS sessions for the purpose of service control and service continuity. The ICS further defines signaling mechanisms between the UE and IMS for transport of information required for service continuity when using CS access for media transport.
IMS as defined by 3GPP supports, in connection with ICS, so-called Single Radio Voice Call Continuity (SR-VCC) where a handover to a CS access is initiated and controlled by ICS. For a proper change of the communication access or domain, such as from a PS domain to a CS domain, several requirements are to be considered. One of the required features is an UE-assisted domain selection for terminating requests. For these cases, in ICS standards, a terminating session setup procedure is defined. In this procedure, it is assumed that the incoming session is delivered over, for example, Gm reference point (signaling interface between the UE and the IMS, which is based on the protocols SIP and SDP) using only PS and the UE decides to use the CS domain. Hence, the session should be set-up by using the CS domain. This may occur, for example, when the UE detects that the PS domain is not capable for audio (speech), i.e. that a corresponding (audio) media flow can not be delivered, so that the UE decides to use the CS domain.
According to the ICS standard, the terminating session setup procedure comprises the following steps. A terminating side Service Centralization and Continuity Application Server (SCC AS) receives a connection initialization message (SIP INVITE message) from a calling UE via a Serving Call Session Control Function (S-CSCF). The SCC AS decides to deliver the call via PS access (PS domain). When the terminating node, i.e. the called UE, receives the INVITE message and detects that the PS domain is not voice capable (e.g. when it is located in a GSM EDGE Radio Access Network (GERAN)), the UE rejects the INVITE. It is further defined in the ICS standard that then the SCC AS sets-up an INVITE towards CSRN (CS routing number, based on which the new call will be routed the CS domain access of the UE), which then gets interworked at an involved Media Gateway Control Function of the called UE (B-MGCF) into a CS SETUP towards the UE.
However, in the present standard, the terminating session setup procedure is not optimized. For example, it is not solved in which way the UE can indicate towards the SCC-AS that the call should be routed to the CS-side of the same UE.
One possible way to solve this is, for example, to let the UE respond with a message indicating that another service is required, such as a SIP 380 response, including an indication in form of an XML indication (extended mark-up language). However, for such an approach, additional standardization work is necessary.